Ultrasonic imaging is a well-known diagnostic medical technique that can generate a visible image of a region of interest (ROI) by transmitting ultrasound into a patient's body, and then receiving and processing reflected ultrasonic echoes, whose characteristics carry information about the insonified body tissues.
In a conventional ultrasonic imaging system, a transmit circuit energizes an array of piezoelectric elements to generate ultrasonic transmit pulses along focused beams into the ROI of the body. Many body tissues of interest are typically nonlinear media such that non-linear distortion is created as the ultrasonic pulses propagate through them. The nature of this distortion indicates characteristics of the tissues that can be extracted for display.
There is a continual effort to improve the quality of these diagnostic images, with “quality” being defined in several different ways. As with many other forms of signal processing, the effort often involves trying to get as much information as possible from the received signals while rejecting as much unwanted signal energy (“noise”) as possible.
One known and common method is often referred to as “harmonic imaging.” As the name implies, instead of processing only the echo signal at the fundamental transmit frequency, this technique involves isolating and extracting information from one or more harmonics of this fundamental frequency either in addition to or instead of at the fundamental frequency alone. Such a technique is disclosed in:
U.S. Pat. No. 6,206,833;
U.S. Pat. No. 7,004,905;
U.S. Pat. No. 7,104,956; and
U.S. Pat. No. 7,513,870.
Echoes reflected from the ROI will typically include a linear component, which is associated with the ultrasonic pulses, and a nonlinear component, associated with the characteristic of tissues and structures of the body. The nonlinear component is extracted from the echoes, and the visible image is generated by processing and analyzing the nonlinear component.
One other known method for improving imaging quality is often referred to as “phase inversion” or “pulse inversion.” This technique exploits the linearity of much of the medium through which the ultrasound pressure waves propagate through the ROI by transmitting and receiving a first ultrasound pulse, and then soon thereafter transmitting and receiving a second ultrasound pulse with a polarity opposite that of the first. Stated differently but equivalently, the two paired pulses differ in phase by 180 degrees. The combined (typically, summed) return signals will therefore “cancel” each other out (sum to zero) with respect to all linear propagation. The remaining signal is then either processed as is, or a harmonic of it is processed to create the displayed image (thereby using both the “phase inversion” and “harmonic imaging” techniques). U.S. Pat. No. 5,632,277 and U.S. Pat. No. 5,951,478 disclose examples of systems that use the “phase inversion” technique.